Kinetic study of oxidation of N-(α-methylbenzylidene) anilines by dimethyldioxirane

Article abstract of DOI:10.1002/kin.20261

A kinetic study of the oxidation of substituted N-(α- methylbenzylidene) anilines by dimethyldioxirane was investigated using a UV/VIS spectrophotometer. Oxaziridines and nitrones were formed as intermediates, and in the excess of dimethyldioxirane corresponding carbonyl compounds, nitrosobenzene or nitrobenzene, were formed quantitatively. The kinetic data were used in the equation for the formation of oxaziridines and nitrones as an intermediate and further oxidation to the corresponding acetophenones and nitrosobenzene. Hammett ρ values were determined for compounds p-substituted on the aromatic ring attached to the carbon atom of the imino group, and it was found that these substituents have very little effect on the oxidation reaction.

Full text of DOI:10.1002/kin.20261

Kinetic Study of Oxidation
of N-( -Methylbenzylidene)
α
Anilines by
Dimethyldioxirane
ˇ
ˇ
ZIGA NOSE, FRANCI KOVAC
Faculty of Chemistry and Chemical Technology, University of Ljubljana, 1000 Ljubljana, Slovenia
Received 19 May 2006; revised 24 January 2007, 19 February 2007; accepted 19 February 2007
DOI 10.1002/kin.20261
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: A kinetic study of the oxidation of substituted N-(α-methylbenzylidene) anilines
by dimethyldioxirane was investigated using a UV/VIS spectrophotometer. Oxaziridines and
nitrones were formed as intermediates, and in the excess of dimethyldioxirane corresponding
carbonyl compounds, nitrosobenzene or nitrobenzene, were formed quantitatively. The kinetic
data were used in the equation for the formation of oxaziridines and nitrones as an intermediate
and further oxidation to the corresponding acetophenones and nitrosobenzene. Hammett ρ
values were determined for compounds p-substituted on the aromatic ring attached to the
carbon atom of the imino group, and it was found that these substituents have very little effect
on the oxidation reaction. ꢀ 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 492–497, 2007
C
formed when a carbon atom of an imine group is ster-
icly hindered [2,3]. The formation of nitrones in re-
actions with peroxyacids was characterized as an ab-
normal pathway for the reaction and the formation of
oxaziridine as normal [4].
INTRODUCTION
Imines as unsaturated compounds can be oxidized by
a variety of oxidizing agents. The first step of oxida-
tion usually results in products known as oxaziridines
or nitrones. The formation of these two compounds
depends very much on the structure of the original
imine, the type of oxidizing agent, and the reaction con-
ditions. Oxaziridines are useful as aprotic-oxidizing
agents, and nitrones are useful for spin trapping in ESR
experiments [1].
Oxaziridines can be prepared by a reaction of imines
with molecular oxygen in the presence of transition
metal complexes [2]. High yield of oxaziridines from
imines was obtained in the oxidation reaction using
an adduct of hydrogen peroxide to urea, known as
urea–hydrogen peroxide (UHP), in combination with
maleic anhydride or other anhydrides. In cases when
aryl groups are attached to the nitrogen atom of imino
group, nitrones are formed preferably [5]. In the oxi-
dation of imines by permanganate, nitrones are formed
[6]. The presented data indicate that water plays a very
important role and a cyclic intermediate is predicted.
It was discovered that some oxaziridines can
be converted to the corresponding nitrones by a
thermal reaction, especially in cases where nitrones are
In the reaction with m-Cl-peroxybenzoic acid, ox-
aziridines are the main products and nitrones are
formed only in a few cases. Nitrones are preferably
Correspondence to: Franci Kovacˇ; e-mail: franci.kovac@fkkt.
uni-lj.si, franci.kovac@gmail.com.
Contract grant sponsor: Ministry of Higher Education, Science
and Technology, Republic of Slovenia.
c
ꢀ
2007 Wiley Periodicals, Inc.

KINETIC STUDY OF OXIDATION OF N-(α-METHYLBENZYLIDENE) ANILINES
493
thermodynamically more stable than oxaziridines. The
oxaziridines can be photochemically converted to
nitrones and vice versa—an important path for the
preparation of optical pure oxaziridines [7].
(1a–g), as shown in Fig. 1, were prepared according to
the literature procedure [9]. The solution of dimethyl-
dioxirane should be dried at least 48 h prior to any
use.
The formation of nitrones in the reactions of C-aryl-
N-alkyl imines with dimethyldioxirane was ascribed
to the electrophilic nature of this oxidizing agent [3].
Overoxidation of C-aryl N-alkyl imines by dimethyl-
dioxirane results in aldehydes and nitroalkanes [8].
The majority of the studies have been done with
imines, where alkyl or aryl groups were attached to
the carbon atom of the imino group and alkyl groups
to the nitrogen atom of the imino group. The oxida-
tion of these compounds by dimethyldioxirane formed
nitroalkanes and the corresponding carbonyl products.
We did not find experimental data for the oxidation
of substituted N-(α-methylbenzylidene) anilines to the
corresponding oxaziridines and nitrones as intermedi-
ates, and further oxidation to carbonyl and nitroso or a
nitro compound especially in view of the kinetic study
of such an oxidation. To our knowledge, this kind of
kinetic data has not been presented so far.
The reaction of the oxidation of substituted N-(α-
methylbenzylidene) anilines (1a–g) with a dry solution
of dimethyldioxirane in acetone was followed by mea-
suring the absorbance at 750 nm to track the formation
and further oxidation of oxaziridine and at 500 nm to
track the formation and further oxidation of nitrone
in a thermostated cell in a UV–VIS spectrophotome-
ter (Varian Cary 100), within the temperature range of
10–50 0.2◦C.
Under experimental conditions, we did not de-
tect any interfering compounds at the wavelengths
used. The reaction mixtures were analyzed by GC/MS
1
(Hewlett Packard HP 6890) and H NMR (Brucker
Avance DPX 300). An example of the GC/MS
chromatogram—from the reaction of the oxidation of
unsubstituted imine—is shown in Fig. 2. We confirmed
the formation of oxaziridines (3a–g) and nitrones
(4a–g) as intermediates.
Further oxidation produced carbonyl compounds
(5a–g) and nitrosobenzene, which was immediately
oxidized to nitrobenzene. We could detect the for-
mation of a nitroso compound when no excess of
dimethyldioxirane was used. We did not detect any
measurable conversion of oxaziridines to nitrones and
vice versa, under our experimental conditions. In all
METHODOLOGY
For our study, we prepared an acetone solution
of dimethyldioxirane according to known procedure
[8] and substituted N-(α-methylbenzylidene) anilines
N
Ph
O
O
X
C
+
H3C
CH3
CH3
1a X=OMe; b X=Me
c X=H; d X=F
2 (DMD)
e X=Cl; f X=CN
g X=NO2
k2
k1
O
C
O
N
Ph
N
Ph
X
C
+
X
CH3
CH3
3 a−g
4a−g
k3
k4
DMD
+
X
COCH3
NO
NO2
5 a−g
Figure 1 Reaction scheme.
International Journal of Chemical Kinetics DOI 10.1002/kin

ˇ
NOSE AND KOVAC
494
Figure 2 Typical GC/MS chromatogram of the reaction mixture.
kinetic experiments, dimethyldioxirane was used in
10-fold excess.
where ksub represents the rate constant for substi-
tuted N-(α-methylbenzylidene) aniline, k0 the rate
constant for unsubstituted N-(α-methylbenzylidene)
aniline, and σ the substituent constant.
The starting concentration of dimethyldioxirane
was approximately 0.1 mol/L. The exact concentra-
tion of dimethyldioxirane was determined before each
kinetic run by oxidation of excess thioanisole (subse-
quent GC/MS analysis) and was in agreement with that
determined using the UV method (λmax ∼ 330 nm and
ε = 10).
RESULTS AND DISCUSSION
In all reactions, substituted N-(α-methylbenzylidene)
The kinetic data were used in Eq. (1)
anilines, oxaziridines, and nitrones were oxidized as
1
intermediates and were confirmed by H NMR and
a · b · (exp(−b · t) − exp(−c · t))
Y =
(1)
GC/MS analysis.
c − b
Maximum concentrations of these two compounds,
under experimental conditions (at 40◦C), were ob-
served for 20 to 30 min. Nitrosobenzene was detected
in all cases when dimethyldioxirane and imines (1a–g)
were used in the ratio 2:1.
Carbonyl compounds (5a–g) and nitrobenzene were
formed quantitatively (96–98%) in 2 h if a 10-fold
excess of dimethyldioxirane was used. The formation
of singlet oxygen in these reactions was not detected.
We found out that the presence of light did not have
a measurable effect on the composition of reaction
products.
where Y is the concentration of an intermediate, t
is time (s), a is the initial concentration of N-(α-
methylbenzylidene) aniline, b is the rate constant for
the formation of an intermediate (oxaziridine or ni-
trone), and c is the rate constant for further oxidation
of an intermediate (oxaziridine or nitrone).
For the calculation of Hammett values, the classical
Hammett equation has been applied [10] (Eq. (2))
ꢀ
ꢁ
ksub
k0
ln
= σ · ρ
(2)
Table I Second-Order Rate Constants for the Oxidation of Substituted N-(α-Methylbenzylidene) Anilines at 40◦C
Oxaziridines
Nitrones
(L mol−1s−1
)
k
(10−2 L mol−1s−1
)
k
(L mol−1s−1
)
k
(10−2 L mol−1s−1
∗
)
∗
∗
∗
X
k
1
3
2
4
a
b
c
d
e
f
0.392 0.025
0.500 0.015
0.518 0.020
0.458 0.028
0.547 0.031
0.749 0.042
–
7.30 0.35
6.45 0.45
6.59 0.23
7.04 0.50
8.22 0.43
9.22 0.55
–
0.72 0.05
0.68 0.03
1.02 0.05
1.11 0.04
1.21 0.05
2.35 0.03
2.05 0.04
2.93 0.15
2.88 0.12
2.37 0.10
2.03 0.05
2.39 0.10
1.33 0.12
0.89 0.05
g
*r2 = 0.93–0.97.
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KINETIC STUDY OF OXIDATION OF N-(α-METHYLBENZYLIDENE) ANILINES
495
Table II Second-Order Rate Constants for the Oxidation of Unsubstituted N-(α-Methylbenzylidenne) Aniline
Oxaziridines
Nitrones
(L mol−1s−1
)
k
(10−2 L mol−1s−1
)
k
(L mol−1s−1
)
(10−2 L mol−1s−1
∗
k
4
)
∗
∗
∗
T (K)
k
1
3
2
283
288
293
303
308
313
323
0.385 0.020
0.455 0.022
0.494 0.020
0.520 0.030
0.572 0.025
0.596 0.040
0.708 0.025
1.69 0.05
1.89 0.05
2.15 0.08
3.33 0.03
4.87 0.10
6.08 0.10
7.07 0.25
–
–
–
0.28 0.01
0.49 0.02
1.38 0.02
1.67 0.03
2.37 0.02
2.69 0.02
0.45 0.01
0.80 0.01
1.02 0.03
–
∗r2 = 0.93–0.97.
Table III Hammett ρ Values at 40◦C
We determined the second-order rate constants for
the first-step oxidation to oxaziridines (3a–g) and ni-
trones (4a–g) and further oxidation to the correspond-
ing carbonyl compounds (5a–g) and nitrobenzene. The
second-order rate constants obtained from UV/VIS,
under pseudo-first-order conditions at 40◦C, are pre-
sented in Table I. The rate constants are an average of
at least two determinations. In all cases, good to excel-
lent correlation was found, and experimental error was
estimated at 5%.
Using the Arrhenius method, we determined the
activation energy (Ea) for the unsubstituted N-(α-
methylbenzylidenne) aniline. For the formation of
oxaziridine, it was 5.2 0.15 kcal/mol and for further
oxidation of oxaziridine it was 7.2 0.15 kcal/mol;
and for the formation of nitrone, we were not able
to determine the activation energy, but for further
oxidation of nitrone the activation energy was 12.5
0.20 kcal/mol. The second-order rate constants are
presented in Table II.
Oxaziridines
Nitrones
∗
∗
∗
∗
k
4
k
k
k
1
3
2
Hammett ρ
0.24
0.09
0.49
−0.34
∗r2 = 0.91–0.94.
A Hammett plot (Fig. 3) of the second-order rate
constants, for the first step oxidation, toward ox-
aziridines and nitrones and for the second step of ox-
idation of oxaziridines and nitrones, to carbonyl com-
pounds and nitrosobenzene, shows fair to good linear
free energy relationship (LFER) against σ constants
(r2 = 0.91–0.94).
The Hammett ρ values are presented in Table III. An
example of UV/VIS kinetic data of substituted N-(α-
methylbenzylidene) anilines oxidation (1a–g) is pre-
sented in Fig. 4.
0.5
0
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
k1
k2
k3
k4
-0.5
-1
-1.5
-2
σ
Figure 3 Hammett plot of the second-order rate constants.
International Journal of Chemical Kinetics DOI 10.1002/kin

ˇ
NOSE AND KOVAC
496
0.30
0.25
0.20
0.15
0.10
0.05
0.00
500 nm
750 nm
0
20
40
60
80
100
Time (min)
Figure 4 UV/VIS kinetic data at 750 and 500 nm at 25◦C for the oxidation of unsubstituted N-(α-methylbenzylidene) aniline
by dimethyldioxirane.
CH3
CH3
methylbenzylidene) aniline is in agreement with the
literature data for the epoxidation of simple olefins by
dimethyldioxirane, where Baumstark and McCloskey
suggested the spiro transition state [11] and Bach et al.
also proposed in their calculations the spiro transition
state [12].
O
CH3
CH3
O
O
O
+
N
N
Ph
p-x-Ph
Ph
p-x-Ph
We believe that the first step of oxidation of N-(α-
methylbenzylidene) anilines, substituted on the aro-
matic ring attached to the carbon atom of imino group,
occurs through a spiro transition state (Fig. 5).
CH3
CH3
Figure 5 Suggested formation of the spiro transition state.
BIBLIOGRAPHY
CONCLUSIONS
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